In situations where small lightweight lasers are required, a desirable configuration is a waveguide laser. If the optical cavity of the laser is bounded by mirrors internal to the waveguide, the laser will be very simple, but also very limited in the manner in which the output may be modulated for use in laser rangefinders or in laser radar. A long standing problem has been to add an intracavity modulator to a waveguide laser with tolerable losses.
It is generally accepted in the art that a waveguide cavity may be extended slightly by displacing a mirror from one end of the waveguide, but that the mirror can be displaced from the end of a waveguide by only one or two characteristic transverse lengths (waveguide diameters or transverse dimension) without incurring a substantial diffractive loss of power from the waveguide mode once propagation out of the guiding structure occurs.
The prior art teaches (Degnan and Hall IEEE Journal of Quantum Electronics Vol. QE-9, No. 9) that waveguide lasers may have a single cavity that is extended substantially beyond the guided region in order to include an intracavity modu-lating element and suffer acceptable optical loss when the displaced mirror focuses the freespace mode back into the waveguide. Such low-loss extnsion of cavity length can be achieved for only a limited range or combinations of mirror radius and guide diameter. For example, an article by Lyszyk et al in Optics Communications, Feb. 1981, p. 327ff shows that for a conbination of 26 cm radius of curvature mirror and 1.5 mm diameter waveguide, a separation of 26 cm between the mirror and the waveguide permits the laser to operate at maximum power, but the power falls by a factor of 2 for a separation distance of 24 cm or 28 cm. If a mirror having a 22 cm radius of curvature is used, the maximum power is half the reference value, for all mirror positions.
If the waveguide cross section is increased, the guide-to-mirror separation for full power is also increased, but the criticality of the separation distance remains. For example, a waveguide diameter of 2 to 3 mm would require a guide-to-mirror distance of at least 50 cm. Such a great extension length would have a significant adverse effect on the mechanical stability of the cavity and thus on the optical stability of the laser.
An article in Optics Communications (Vol. 4, No. 4, page 283 (1971) by Bjorkholm et al discloses the use of two freespace coupled cavities with planar mirrors, one of which contains a gain medium and the other of which is a Fabry-Perot etalon providing frequency control by means of a grating. Those skilled in the laser art would expect, in view of the above experiments and the state of the art, that a combination of waveguide gain medium and planar mirrors (or grating) would be possible only if the guide-to-grating distance were not more than several waveguide diameters. However, U.S. Pat. No. 4,241,319, issued to Aristotle Papayoanou on Dec. 30, 1980, suggests that a waveguide laser may be used with a Fabry-Perot etalon that is sufficiently long to accomaodate a Stark cell used to tune and/or to modulate the laser. The patent indicates that diffraction losses can be a problem, but does not disclose a comparison of power levels for that laser and a laser without an extended cavity.
For a device that must be used in the field and so has power and weight limitations, as well as stringent stability requirements, active modulation of the type shown in the '319 patent is not attractive.